Most dishwashers are configured so that the bottom rack holds pots, pans and heavier items, while the top rack accommodates glasses. Usually, when dishes come out of the dishwasher still dirty, they were washed on the top rack. This may occur because of dishwasher malfunction or human error. Your glasses also could be suffering from your water type.
Improper Loading in General
Dishwashers usually come with loading recommendations meant to maximise efficiency without blocking the essential functions of your appliance. The first rule to remember when loading dishwashers is never block the water jets. The jet arms must be allowed to turn freely and raise into the dishwasher during wash cycles. Do not overload your dishwasher. Overloading or loading that blocks water jets' functions prevents the most powerful streams of water and soap from reaching the top rack. This cuts down significantly on the quality of washing glasses on top rack receive, and can leave the glassware dirty or stained with soapy residue.
Improper Loading of Glasses
Even if the rest of your dishwasher is loaded correctly, improperly stacking glasses can leave the items dirty. You should always situate glasses around rack tines, not over them. Putting the glasses over tines blocks full water and soap access, leaving dirt or residue behind.
Failure to Rinse Glasses
Certain beverage remnants are particularly difficult to remove, especially if the glasses have sat for several days in the dishwasher. Liquids, including coffee, tea and wine, should always be pre-rinsed from glasses before putting the glasses into the dishwasher.
Hard Water Deposits
Hard water deposits can leave glassware looking dirty or spotted after a dishwasher cycle. The deposits appear as white film, cloudiness or spots. To properly diagnose your dirty glasses with hard water marks, soak the glasses in undiluted white vinegar for five minutes. Rinse the glasses with water and dry. If the stains are gone, hard water is likely to blame. You can prevent hard water deposits on your glassware by using more detergent, using the hottest water temperature settings on the dishwasher, raising the hot water heater temperature or adding a rinse aid to the dishwasher.
Soft Water Etching
Soft water can also cause problems for glasses in dishwashers, although the problems can quickly become permanent, unlike hard water deposits. Your glasses may appear to have a white film or spots, or change colour in certain areas. Submerge the glasses in undiluted white vinegar for five minutes, rinse with water and dry. If the film or spots did not go away, your glasses were likely etched in the dishwasher by soft water, causing permanent cloudiness or a dirty looking film. This doesn't mean your glasses are unclean, they just look so.
A problem with your dishwasher also could cause your glasses to come out of the cycle dirty. Clogs in the water jet arms prevent the jets from operating at full power. The top rack items, including glasses, always suffer more from clog effects, thanks to gravity. Try cleaning out the water jet holes with a safety pin or toothpick to see if the problem goes away. Alternatively, your dishwasher's water inlet valve could be broken. The valve supplies the dishwasher with water, and a broken piece prevents the full power of the water from coming through. Once again, the top rack items are more likely to suffer from the decreased water flow.